Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or other multiple access techniques. Such systems can conform to standards such as Third-Generation Partnership Project 2 (3gpp2, or “cdma2000”), Third-Generation Partnership (3gpp, or “W-CDMA”), or Long Term Evolution (“LTE”). In the design of such communications systems, it is desirable to maximize the capacity, or the number of users the system can reliably support, given the available resources.
One technique used to enhance performance, including system capacity and data throughput, is to lower the required transmit signal power by employing transmit diversity. Transmit diversity involves transmitting data on two or more antennas, where the geographical separation between the antennas leads to path loss characteristics that are independent from antenna to antenna. A receiving station can coherently combine signals from the transmit diversity antennas, and the noise introduced in the channel will not combine coherently, thus increasing the signal-to-noise ratio (SNR) received.
In some closed-loop transmit diversity schemes, an example of which is proposed in the aforementioned W-CDMA specification, a mobile station sends phase adjustment information to the base station to adjust the phase of the signals being transmitted on one or more antennas. The phase can be adjusted such that when the signals from the various transmit antennas are received at the mobile station, they combine coherently. To signal the phase adjustment information to the base station, the mobile station may encode the information using a code, such as a two-bit code that uniformly quantizes the 360° range of possible phase adjustments into four levels. While such coding techniques may be simple to design and implement, they may inefficiently use the over-the-air signaling resources under certain channel conditions, e.g., static or quasi-static channel conditions, wherein any phase adjustments are expected to vary slowly over time.
It would be desirable to provide simple and efficient techniques for encoding the phase adjustment information sent on a feedback channel for closed-loop transmit diversity and other system employing feedback signaling.